The present invention relates to computer animation. More specifically, the present invention relates to methods and apparatus for preserving the shape of simulated objects using techniques for velocity dragging.
In computer graphics imagery, motions and positions of secondary objects, such as hair, clothing, and plants are usually too complex for a human animator to directly control at every stage of a computer animation. Instead, the human animator specifies the physics and/or physical properties of the secondary or simulated objects. A computer program then employs physically-based numerical methods and techniques to simulate the motions and positions of the secondary objects over time based on the physics or physical properties of the individual secondary objects.
For simulated clothing objects, for example, the animator specifies the physical properties and construction of the cloth. For example, the animator specifies how the cloth bends due to forces or collisions with solid objects. The animator further specifies how the cloth deforms or collides with itself. Moreover, the animator specifies external forces that act on the cloth, such as gravity and wind.
In addition to modeling the physical properties of the simulated objects, the animator specifies motions and positions of kinematic or non-simulated objects (e.g., characters upon which the clothing objects rest). The animation of a non-simulated object generally is independent of and otherwise unaffected by motions and positions of simulated objects. However, the motions and positions of the non-simulated objects often are the principal influencer of motions and positions of simulated objects, as clothing and hair are likely to be associated with a kinematic character.
Consider a computer animation of a human character standing upright, wearing a jacket. The human character is a kinematic or non-simulated object that is directly animated by the skilled human animator. The animator specifies the physics (e.g., the physical properties) of the jacket which is a simulated object. In addition, the animator models how the jacket is associated with and worn by the human character. During simulation, the computer program simulates the motions and positions of the jacket using physically-based numerical techniques in response to external forces and the motions and positions of the human character.
If the physical properties and external forces acting on a simulated object are accurately modeled, the resulting motion of the simulated object will be plausible and seemingly realistic. In our jacket example, the cloth of the jacket should hang down and fold naturally. Furthermore, the cloth should react according to the motions and positions of the human character when the human character wears the jacket. However, modeling the simulated objects to be truly accurate is a delicate balance between the limitations and complexities of the animator's knowledge of physics and particle systems on the one hand and budgetary and time constraints on the other.
In addition, other problems exists with physically-based numerical methods and techniques used in computer animations. A particularly difficult problem in the simulation of secondary or simulated objects, such as cloth, is dealing with creeping or oozing behaviors. A creeping or oozing behavior occurs when motion of a simulated object associated with a non-simulated object continues in a visually unpleasing manner after a change in motion of the non-simulated object.
In the real world, most garments such as shirts, jackets, or pants undergo no significant movement or change in shape when their wearers cease moving. Internal forces in clothes, and friction between the clothes and their wearer, generally lock the clothes into a fixed position when the wearer's motion ceases. Typically, the clothes stops moving far less than a second after the wearer does. Although some garments, such as long dresses or ties, typically swing back and forth for some period of time, this desired and realistic motion is different from undesirable creeping or oozing behaviors that result during simulation using physically-based numerical methods and techniques.
Creating simulation programs for simulated objects, such as cloth, that can achieve the same effect after the wearer ceases moving has been difficult. One solution is to instruct the computer program during the simulation of our jacket example to freeze the cloth of the jacket in place whenever the human character ceases moving. The cloth then would be allowed to move again, when the human character begins to move. A problem with this solution is that our human character rarely remains exactly motionless. Typically, even when an animated character ceases movement, some small amount of “keep-alive” motion is applied. For example, the animator may rotate limbs of the character a few degrees or have the character sway back and forth just a little. It is during keep-alive motion, that the creeping or oozing of simulated objects is most apparent.
Accordingly, what is desired are improved methods and apparatus for solving the problems discussed above, while reducing the drawbacks discussed above.